The need to remain cost and performance competitive in the production of semiconductor devices has driven the industry to a continuing increase in device density with a concomitant decrease in device geometry. To facilitate the shrinking device dimensions, new lithographic materials, processes and tools are being considered. Currently, 248 nm lithography is being pursued to print sub 200 nm features. To do this, tools with higher numerical aperture (NA) are emerging. The higher NA allows for improved resolution but reduces the depth of focus of aerial images projected onto the resist. Because of the reduced depth of focus, a thinner resist will be required. As the thickness of the resist is decreased, the resist becomes less effective as a mask for subsequent dry etch image transfer to the underlying substrate. Without significant improvement in the etch selectivity exhibited by current single layer resists, these systems can not provide the necessary lithography and etch properties for high resolution lithography.
Another problem with single layer resist systems is critical dimension (CD) control. Substrate reflections at ultraviolet (UV) and deep ultraviolet (DUV) wavelengths are notorious to produce standing wave effects and resist notching which severely limit CD control of single layer resists. Notching results from substrate topography and nonuniform substrate reflectivity which causes local variations in exposure energy on the resist. Standing waves are thin film interference (TFI) or periodic variations of light intensity through the resist thickness. These light variations are introduced because planarization of the resist presents different thickness through the underlying topography. Thin film interference plays a dominant role in CD control of single layer photoresist processes, causing large changes in the effective exposure dose due to a tiny change in optical phase. Thin film interference effects are described in “Optimization of optical properties of resist processes” (T. Brunner, SPIE Proceedings Vol. 1466, p. 297, 1991), the teaching of which is incorporated herein by reference.
Bottom anti-reflective coatings or BARCs have been used with single layer resists to reduce thin film interference. However, these thin absorbing BARCs have fundamental limitations. These materials are generally spin applied. The thickness of the BARC and the resist can not be controlled to the accuracy required to operate at the target thickness to achieve minimum reflectance. The resist thickness may also vary due to existing topography. Thin underlying films such as silicon nitride or silicon oxide tend to exhibit some thickness nonuniformity after deposition. The thin BARC will generally not effectively planarize this thin underlying films. Thus, as a result there will be a variation in exposure energy into the resist. Current trends to reduce topography via chemical/mechanical polishing still leave significant variations in film thickness over topography.
Vapor deposited such as plasma enhanced chemical vapor deposited PECVD BARCs are currently being investigated. We consider an example with a carbon ARC deposited by PECVD process. FIG. 1 represents a swing curve comparison of BARC/single layer resist process (a) and a bilayer resist process (b) in which a thin resist on the order of 2000 A is applied on top of a thick underlayer. These results are obtained by simulations on two substrates, silicon and SiO2. Swing curve reflectance at 248 nm as a function of resist thickness is computed at the resist/air interface. The simulated structure of the single layer resist (FIG. 1a solid line) includes a Si substrate, 900 A thick bottom ARC with n=1.9 and k=0.42 at 248 nm and photoresist with n=1.8 and k=0.02 at 248 nm. The optical constants of the bottom ARC are typical of PECVD ARCs. FIG. 1a shows that by using 900 A thick bottom ARC with n=1.9 and k=0.42 about 2% swing ratio can be achieved on a silicon substrate.
Similarly, a simulated bilayer structure on silicon and on a SiO2 layer is shown in FIG. 1b. This structure includes a Si substrate, a 6000 A thick underlayer with n=1.8 and k=0.2 at 248 nm and a silicon containing resist with n=1.78 and k=0.01 at 248 nm. Swing ratio of less than 4% similar to the thin ARC process was obtained for the bilayer resist structure on a Si substrate (FIG. 1b solid line). To demonstrate the dependence of the ARC on underlying topography, 500 A of SiO2 was deposited on the silicon substrate (FIG. 1, dotted lines). As can be seen in FIG. 1a, the single layer resist structure is very dependent on the underlying substrate reflectivity and topography, whereas, the bilayer structure was independent of underlying topography. A large variation in reflectance, about 18% swing ratio, which directly translates to CD variation was observed for the SLR structure with an oxide underlayer (FIG. 1a dotted line). This reflection variation directly corresponds to CD variation, and it's about 18% as a function of the resist thickness and does not meet +−10% linewidth control criterion even on single layer substrate. In contrast, almost no reflectance variation was observed for bilayer (FIG. 1b dotted line). Swing ratio of less than 4% was obtained for the simulated bilayer resist structure independently from the underlying substrate reflectivity. The entire range of the data easily fits within the +−10% linewidth control criterion. So from an optical point of view, bilayer resist structure appeared to be more beneficial compared to SLR structure.
To overcome some of the limitations of single layer resists, multilayer resist systems have been investigated such as bilayer resists. In a bilayer structure (FIG. 2), a first thick bottom polymer layer with suitable absorption at the exposing wavelength is spun on the substrate. This layer serves to planarize the underlying structure, minimize TFI and to dampen substrate reflections (notching). On top of the underlayer is applied a thin resist (on the order of 1000-4000 A). The resist generally contains Si functionality for etch resistance. The resist is exposed and developed to form a pattern in the resist. This pattern is then transferred to the underlayer by plasma etching (the si containing resist acts as hard mask for the etch process). An extensive review of thin film imaging techniques which include multilayer resists, top surface imaging (CARL) can be found in “Polymeric silicon-containing resist materials”, R. D. Miller and G. M. Wallraff, Advanced Materials for Optics and Electronics, Vol. 4, 95-127 (1994), W. Moreau, Semiconductor Lithography, 1988, Plenum, Chapter 12, pg. 591, D. Seeger, IBM J. Res. Dev. (USA) Vol. 41, N. 1-2, (1997) and S. Hien, SPIE proc. Vol. 3333 154-164 (1998) which are incorporated herein by reference.
TSI processes differ from bilayer in that a typical non silicon containing CA resist is used. After the resist is exposed, it is silylated by vapor or liquid silylation techniques. The silylated resist in turn acts as the etch resistant hard mask for transferring the pattern from the resist to the underlying substrate. A bilayer configuration utilizes a silicon containing resist; it requires no external silylation process. Both bilayer and TSI require an appropriate underlayer. A bilayer resist and TSI resist structure offers significantly higher aspect ratio than can be achieved with single layer resists. Single layer resists are on the order of 5000 A-6000 A.
A bilayer structure uses a thin resist (1000-4000 A) which can provide significant improvement in resolution and process latitude. The underlayer is on the order of 1000 A-10 um most preferably 4000 A-2.0 um, therefore once the pattern is transferred to the underlayer, high aspect ratio is attained. The thickness of a bilayer structure also offers significant advantages for etch. In single layer resists, microchannels often form through the resist after etch. When the resist is thin, the microchannels extend into the underlying films creating defects. Because of the increased thickness of the bilayer stack, any microchannels formed in the resist do not penetrate below the underlayer.
The most commonly used underlayer in a bilayer structure has been a novolac/diazonapthoquinone resist as found in “Bilayer resist approach for 193 nm lithography”, Schaedely et al., Proc. SPIE—Int. Soc. Opt. Eng. (USA) Vol. 2724 1996, p 344-54. It has been believed that the lithographic performance of a bilayer resist was strictly governed by the resist. The underlayer was not believed to have a significant role in the lithographic performance.
In contrast, herein it is found that the underlayer plays a significant role in impacting the lithographic performance of the bilayer and TSI resist structure. It is found that using a typical novolac based underlayer results in an interfacial reaction with the imaging resist, resulting in significant residue and limiting the ultimate resolution.
It is therefore desirable to have a tuned underlayer material which does not interact with the resist and provides a bilayer or TSI structure with high resolution. Herein, bilayer resist and TSI resist structures are described which contain underlayers whose optical, chemical, and physical properties have been tuned to result in a high performing structure. Methods of tuning the underlayer properties are described. In addition, new materials which can be used as suitable bilayer underlayers are also described.